Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy customers requirements resulting in a large loss of profits. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying thermal energy to the dye donor material, to transfer dye to the thermal print media, thereby forming an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, print-head, and vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media, in roll form, from the material assembly or carousel. The thermal print media is measured and cut into sheet form of the required length and transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. A length of dye donor material, in roll form, is metered out of the material supply assembly or carousel, measured and cut into sheet form of the required length. The dye donor material is transported to and wrapped around the vacuum imaging drum, such that it is superposed in registration with the thermal print media. The translation drive, part of the scanning subsystem, traverses the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media
The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the thermal print media in accordance with an information signal. The printhead includes a plurality of optical fibers coupled to the laser diodes at one end and at the other end to a fiber optic array within the printhead. The printhead moves relative to the longitudinal axis of the vacuum imaging drum and dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead to the dye donor material, is converted to thermal energy in the dye donor material.
Although the image processing apparatus described is satisfactory, it is not without drawbacks. Obtaining the correct printhead focus requires an iterative sequence of precise manual adjustments using a micrometer and generation of focus prints that provide feedback information on focus accuracy. Measurements from the test print are used to determine whether or not further adjustment is necessary. For this reason, printhead focus requires trained service personnel to calibrate printhead components and make repeated manual adjustments. This limits the ability of the image processing apparatus user to adapt the machine to media having different thickness or to media having different spot focus requirements. Similarly, the angle of the printhead about its axis, which determines the distance between imaged dots, described in U.S. Pat. No. 5,164,742 (Back, et al.), also requires precise manual adjustment, with a series of test prints for feedback, and one or more manual readjustment cycles. It would be advantageous to automated control of these adjustments.
Conventional servo loops are one way to solve the problem. For example, stepper motors are widely used in optical equipment to focus lens assemblies automatically and a number of commercially available "point-and-shoot" SLR cameras employ stepper motors to obtain correct focus. U.S. Pat. No. 5,047,796 (Tagami et al.) discloses a stepper motor for obtaining camera focus. However, servo loops using stepper motors are prohibitively expensive and impractical for controlling head focus or angular positioning.
Stepper motors are inherently well suited to applications that require precision positioning. The construction of the stepper motor provides a set of discrete, fixed positions, shaft based on a symmetric arrangement of rotor poles and stator windings. U.S. Pat. No. 5,453,777 (Pennsavecchia et al.) discloses multiple stepper motors for adjusting focus in a laser imagesetter apparatus. Focus is adjusted individually for each channel, each of which writes with a single laser focused through its own lens assembly. The device, however, generates a swath in multiple passes, unlike the continuous-swath generated by the image processing apparatus of the present invention.
Methods for homing or registration using a stepper motor employ phase state relationship of currents in combination with a microswitch or other sensor that indicates proximity of the driven device to a home position. U.S. Pat. No. 4,394,696 (Yoshimaru) uses control logic for positioning a magnetic read-write head in a tape drive and uses a microswitch transition to indicate that the head is in the neighborhood of its home position. When a control circuit senses this transition, the phase state relationship of currents is used to move the read-write head to home.
In much the same way, U.S. Pat. No. 5,491,595 (Alsborg, et al.) discloses a positioning method for a magnetic read-write head using a proximity sensor and a control algorithm that provides positioning using a coarse-fine sequence. First, control logic drives the stepper motor a number of steps (N) at a time in a first direction, while checking the proximity sensor for a transition indicating coarse position. Control logic then drives the stepper motor a number of smaller steps in the opposite direction, past the transition point. Next, control logic drives the stepper motor, again in the first direction, a number of still smaller steps (M&lt;N) at a time, until the sensor transition recurs, indicating that fine-tuning has been obtained.
U.S. Pat. No. 4,264,220 (Okcuoglu et al.) discloses running a stepper motor to home a print wheel, where a stop position on the print wheel may initially be in any angular position relative to a corresponding fixed stop element. In order to compensate for possible worst-case print wheel positioning, where the print wheel must rotate over its full angular travel path in order to reach home position, the stepper motor rotates the print wheel the maximum possible number of steps to make sure that the print wheel stops in the fixed stop position. This often means running the motor blocked for at least some number of steps, which is satisfactory for many types of stepper motors. On less expensive motors, the internal mechanical configuration of stepper motor components may not withstand running with the rotor blocked for extended periods. Notably, some types of stepper motors widely used for linear positioning have plastic internal components where, for example, a plastic rotor rotates about a metal lead screw. Running such a motor with blocked rotor can cause damage to plastic threads and ruin the motor. This limits the use of such motors when used with mechanical stops for homing applications.
U.S. Pat. No. 4,395,742 (Ostroff) discloses a method for homing a magnetic read-write head in a disk drive that uses, in combination, a mechanical stop and the phase relationship described above. By stopping head movement mechanically, then using a known current phase relationship, or "program," a method is disclosed for accurate homing of the magnetic read-write head upon power-up. During normal seek operation, the stepper motor is driven at higher current levels. To prevent mechanical damage due to running with the rotor blocked, the motor is run with reduced current and therefore with reduced torque for homing the magnetic read-write head. This solution is acceptable for applications that allow movement with low motor torque, such as positioning a light-weight read-write head, however, it is not adequate for focusing or angular positioning for a scanning printhead, since this movement requires the full torque available from the stepper motor. Further disadvantages of this approach include the added complexity and cost of circuitry for setting the alternate current level.
U.S. Pat. No. 4,408,907 (Bernardis) discloses use of a stepper motor to adjust the angle of a printhead in a dot-matrix printer. The apparatus disclosed sets the printhead angle to one of two possible positions, however, there is no capability provided for any fine-tuning of printing head angle and the stepper motor is used merely to toggle the printing head to either of two angle settings.
Accurately setting the swath width for a multichannel printer requires mechanical adjustment within tight tolerances (see U.S. Pat. No. 5,083,143 (Hoffman) which describes swath width adjustment in an inkjet printer.) Repeated cycles of manual adjustment and generation of a test print can make printhead angle adjustment a time-consuming and costly procedure. For a printhead requiring high resolution, this adjustment typically requires use of a microscope, micrometer, or other sensitive instrumentation. For example, U.S. Pat. No. 5,146,242 (Zielinski) describes a method for manual adjustment of a printhead angle in a multichannel apparatus using micrometer screw adjustment. Without automatic adjustment of the head angle in an image processing apparatus, the apparatus is limited to imaging at one specific resolution once head angle adjustment is obtained.
There is a need for a method to automatically adjust printhead angle or focus or both as the test print is generated.